It is well known to locate a spring between the carbon brush and a closed end of the brush box for resiliently urging the carbon brush outwardly through an open end of the brush box into contact with the motor's commutator. It is also known to have a longitudinal slot in a side of the brush box through which slot a flexible braided brush lead extends and can move with the brush relative to the brush box. The flexible brush lead penetrates directly into the brush and is secured therein by a delicate and time consuming manufacturing process.
U.S. Pat. No. 3,339,098 discloses a carbon brush assembly in which a rectangular sectioned brush has formed at its inner end a cylindrical portion over which is fitted an annular, metal connecting element. This connecting element has an extension which extends transversely outwards through a slit along a side wall of the brush box. A flexible brush lead is attached to this extension which is movable along the slit as the brush wears. A coil spring engages over the cylindrical portion of the brush and is compressed between the annular connecting element and a closure cap closing the rear end of the brush box. Shoulders are formed internally of the brush box to limit forward movement of the annular connecting element. The spring functions both to ensure electrical contact between the annular connecting element and the carbon brush, and to press the carbon brush against the commutator.
This arrangement has several disadvantages. The cylindrical portion of the brush has to be produced by a separate manufacturing operation, carbon brushes generally being of rectangular or square cross-section.
Furthermore, it is only possible to assemble the carbon brush, spring and brush lead connecting element in the brush-holder if the slit along the brush-holder remains open at least at one end during assembly; otherwise, it would be impossible to introduce the connecting element into the interior of the brush-holder. It is consequently necessary, after inserting the carbon brush, connecting element and spring into the brush-holder, to close either the rear end of the brush-holder or the front end of the slit provided in the brush-holder; this necessitates an additional assembly operation. Also, the contact pressure which is exerted by the spring upon the connecting element, and whereby the electrical contact between connecting element and carbon brush is maintained, is comparatively weak; this is because the force of the spring is chosen in accordance with the desired contact pressure of the carbon brush against the commutator and is required to be relatively small.